1. Field of the Invention
The invention relates to fiber-reinforced composites and, more specifically, to composites having fibers dispersed through a thermoset or ceramic matrix wherein the matrix is formed by curing or pyrolyzing a linear polymer having repeating units of carborane, siloxane or silane and acetylene.
2. Background of the Related Art
Fiber-reinforced composites, for example, carbon-carbon composites having carbon fibers embedded in a carbonaceous matrix, are strong, lightweight materials that have many uses, including as structural components in missile, aircraft and aerospace vehicles.
A drawback to using fiber-reinforced composites in harsh environments is that the matrix or the fibers may oxidize and erode when exposed to high temperatures, causing a breakdown and failure of the composite material. Efforts have been made to improve the oxidation resistance of fiber-reinforced composites by creating a barrier coating over the material or by adding compounds containing boron and silicon or other refractory elements to the fibers or the matrix. For example, U.S. Pat No. 4,668,579 to Strangman et al describes a carbon-carbon fiber composite that is protected from oxidation by having a protective underlayer of boron carbide and a protective overlayer of silicon carbide deposited onto the fibers by chemical vapor deposition (CVD). Under oxidating conditions, the boron carbide and silicon carbide are converted into boron oxide and silicon dioxide. U.S. Pat. No. 4,894,286 to Gray describes an oxidation-resistant carbonous composite having boron, silicon and titanium elements or compounds dispersed throughout the structure. The boron, silicon and titanium are added in the form of powders to the precursor resin that becomes the carbonous matrix. When the composite is exposed to high temperatures in an oxidizing environment, the boron, silicon and titanium serve as oxygen getters to protect the composite from oxidation. The boron, silicon and titanium are converted into oxides to form a sealant glass that fills microcracks in the composite. In each of the above composites, the effectiveness of the thermal protection depends on how well or how thoroughly the protective elements can be dispersed throughout or coated onto the composite.
It is an object of the invention to provide an oxidation resistant fiber-reinforced composite that has protective components coated onto the fibers and homogeneously distributed throughout the composite. It is a further object of the invention to provide an oxidation resistant fiber-reinforced composite made from materials that are easily processed.
It has recently been discovered that boron-containing, silicon-containing and carbon-containing moieties can be combined into linear inorganic-organic hybrid polymers that can be used to make oxidatively stable thermosets and ceramics. Such linear inorganic-organic hybrid polymers, specifically, carborane-siloxane/silane-acetylene polymers and their corresponding thermosets and ceramics, are disclosed in U.S. Pat. Nos. 5,272,237; 5,292,779; 5,348,917 and 5,483,017, in the U.S. Patent Application entitled "Thermoset Polymers Made by Blending Poly(carborane-siloxane/silane-acetylene) and Poly(siloxane-acetylene)" filed Nov. 13, 1996, and in the U.S. Patent Application entitled "Improved Method for Synthesis of Linear Inorganic-Organic Hybrid Polymers" filed as a Statutory Invention Registration on Nov. 27, 1996. Polymers that include carboranyl, silyl or siloxyl and acetylenic groups in the same polymeric chain combine the desirable features of both inorganics and organics: the carborane groups provide thermal and oxidative stability, the silane or siloxane groups provide additional thermal stability and chain flexibility and the acetylenic groups allow cross-linking of adjacent polymer strands to form thermosets. Carborane-silane/siloxane-acetylene polymers have the advantage of being extremely easy to process and convert into thermosets or ceramics since they are either liquids at room temperature or low melting solids and are soluble in most organic solvents.